At the present time, the implementation of a sustainable development strategy makes it possible to increase productivity, cost benefit and profitability by using, for example, by-products derived from other industries.
Thus, within the framework of the preservation of natural resources, it is interesting to replace natural sodium sulphate (also known as Glauber's salt) by synthetic sodium sulphate, especially in countries where the resources are dwindling or are non-existent.
Yet, various industries lead to sodium sulphate as a by-product, which may be a new and inexpensive source of raw material.
Phosphogypsum is the by-product resulting from the production of phosphoric acid by attack of phosphates by sulphuric acid.
Phosphogypsum is generally constituted of sulphates, calcium, different impurities such as P2O5, F— and organic substances (phosphoric acid, phosphates, fluorites), which adhere to the surface of the crystals of phosphogypsum or may substitute them in the matrix.
The quantity of phosphogypsum produced is very great: the production of one tonne of phosphate is accompanied by the production of five tonnes of phosphogypsum. The annual worldwide production of this material is currently estimated at 250 million tonnes [5].
Processes for producing sodium sulphate from sulphuric acid and sodium hydroxide or sodium chloride are well known. Nevertheless, these processes are costly and complicated. Furthermore, they are hazardous because they resort to toxic products.
It is also known from the document WO 97/16376 [3] to recrystallize sodium sulphate recovered from scale derived from the treatment by sodium bicarbonate of fumes from glass furnaces. However, this process consumes water and leads to the discharge into the natural environment of large quantities of saline water comprising for example chlorides, sulphates, fluorides.
The document EP 0 001 533 [4] describes a process of purification of sodium sulphate, notably sodium sulphate containing traces of methionine. The purification is carried out in oxidising medium by a solution essentially of the group of chlorates and persulphates, then the sodium sulphate thereby treated is calcinated.